Not applicable
The invention pertains to optical communications and in particular to the control of optical beams using adaptive optical elements.
The field of communications has benefited enormously from the introduction of optical communications technology. Fundamentally, this technology exploits the inherent bandwidth potential of the light itself as a carrier for communications signals. As this technology matures, the need for the direct optical processing of the signals is becoming greater. Much of the communications infrastructure in operation in the field today relies on signals being converted from optical form back to electrical form for much of the signal processing and management. Direct optical processing has the benefit of avoiding the need for optical to electrical and electrical to optical conversion equipment with its associated costs, losses, and amplification requirements.
One of the critical issues within the field of optical communications relates to the situation where many optical signal channels on parallel fibers have to be controlled, adjusted, or switched at a single point in the communication system. This issue creates a corresponding need for a microelectronic device with a considerable level of device integration and individually adjustable channels. Simultaneously there is a clear need for devices that will perform these functions while being rapidly adjustable in operation. It is also desirable for candidate devices to have relatively low insertion losses and a minimum possible wavelength dependence.
One of the fundamental building blocks of an optical communications system is the optical cross-connect or optical crossbar switch. Optical crossbar switches function to selectably connect any one of an array of incoming optical signals to any one of an array of outgoing channels. Inherently these devices consist of a multiplicity of optical communications channels which may be implemented on a semiconductor wafer using micro-machining technology.
A variety of specific individual device structures have been proposed and fabricated to address the above-described application. Many of these devices rely on non-linear Optic materials to obtain switching actions. Another popular way to address the above described application is by means of micro-electromechanical structures. These micro-electromechanical structures are usually micro-mirror devices that tilt, flex, or flip upon application of an appropriate control voltage.
Most typically, these devices have two states, one of which causes an incoming beam of light to bypass the mirror, either by flipping the mirror down or removing it from the beam path by some other means, and a second position in which the mirror is interposed in the path of the beam so as to reflect it into some or other desired direction. This is done in order to couple the optical beam into an output channel, usually via a micro-lens and optical fiber arrangement.
The small apertures involved in the light-carrying cores of the optical fibers, particularly single mode fibers, lead to considerable beam divergence. Divergence is typically addressed by using suitably small micro-lenses that seek to collimate or focus the divergent light beam emerging from the input signal optical fiber. At the output end of a crossbar switch there is a corresponding requirement for a lens to ensure appropriate coupling of the optical beam to the output optical fiber. Again, there are great constraints on the scope of the physical dimensions of these devices.
A particular problem in these arrangements is the fixed nature of the micro-lenses which restricts the latitude of design available to optical engineers. It also puts constraints on the silicon micro-machined optical switching devices that typically form the heart of these crossbar switches, in that the optical switching devices have to be fabricated such that they are optically matched to the fixed lenses in order to ensure minimum insertion losses and to restrict losses inside the devices.
These design restrictions would be reduced if suitable adaptive micro-lenses were available. Since one of the strengths of optical communications is the very wide bandwidth that it makes possible, there is every incentive to ensure that the optical devices and elements that are part of optical crossbar switches are commensurately fast, as this determines the rate at which routing and managed networking of the communication signals may be achieved. This issue applies not only to the sophisticated silicon devices in a crossbar switch, but also to any adaptive micro-lenses within such a crossbar switch.
Liquid crystal lenses to address some of these issues are known in the art. However, these devices have limited speed due to the inherently slow switching speed of the liquid crystal mechanism. Over the past decade, much collective effort was devoted to deformable macroscopic mirror devices for light projection systems, and in this respect piezoelectrically deformed lenses are known, but these clearly do not lend themselves to application in miniaturized optical crossbar switches.
Micro-electromechanical (MEMS) devices have been applied in the field of adaptive optical devices before and are attractive from the point of view of their relatively high switching speeds. However, MEMS devices are more typically employed as two state devices for binary functions, this being due to the difficulty in obtaining controlled analog deformation from the cantilever and torsion structures typically employed in these devices. Devices aimed at the controlled adaptation of light beams are therefore typically difficult to fabricate using typical prior art MEMS devices.
In general, it is preferable for an adaptive optical element to maintain its full dynamic range of adaptation, while simultaneously providing acceptable control over that range, most particularly, at the low end of the adaptation range. The concern related to the low end of the adaptation range is due to the fact that there are many optical systems in which slight adaptation of focal lengths and the like, may result in greatly disproportionate effects within the overall optical systems.
Another approach for providing adaptive optical elements involves providing a membrane that is fixed at its perimeter, or that extends over a system of holes, and then deforming one or more of these membranes using an electric field for electrostatic attraction. The typical device fabricated in this way may be used to produce beam extinction or modulation by employing very tiny deformations together with the principle of optical interference. Along with these general principles of operation, comes a general tendency of these devices to be inherently wavelength-sensitive.
Individual elements in a micro-electromechanical array of integrated stretched membrane devices are independently addressed and controlled to produce independently controlled degrees of refraction of light beams.